feat: add admin publishing workflow and yar theme

Add Go/Postgres admin APIs, Angular admin UI, manual build flow, asset uploads, markdown import/export, configurable slug generation, and the Yar reading theme. Exclude local docs and generated development artifacts from version control.

backend/internal/admin/exporter.go

@@ -0,0 +1,135 @@
+package admin
+
+import (
+	"bytes"
+	"context"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+	"time"
+
+	"gopkg.in/yaml.v3"
+)
+
+type Exporter struct {
+	postsDir string
+}
+
+type postFrontmatter struct {
+	ID          string   `yaml:"id"`
+	Slug        string   `yaml:"slug"`
+	Title       string   `yaml:"title"`
+	Summary     string   `yaml:"summary"`
+	Status      string   `yaml:"status"`
+	Tags        []string `yaml:"tags"`
+	Cover       string   `yaml:"cover"`
+	Version     int      `yaml:"version"`
+	SlugSource  string   `yaml:"slug_source"`
+	SlugLocked  bool     `yaml:"slug_locked"`
+	PublishedAt string   `yaml:"published_at"`
+	CreatedAt   string   `yaml:"created_at"`
+	UpdatedAt   string   `yaml:"updated_at"`
+}
+
+func NewExporter(postsDir string) *Exporter {
+	return &Exporter{postsDir: postsDir}
+}
+
+func (e *Exporter) ExportPublishedPosts(ctx context.Context, posts []Post) error {
+	if err := os.MkdirAll(e.postsDir, 0o755); err != nil {
+		return fmt.Errorf("create posts dir: %w", err)
+	}
+
+	publishedFiles := make(map[string]bool, len(posts))
+	for _, post := range posts {
+		select {
+		case <-ctx.Done():
+			return ctx.Err()
+		default:
+		}
+
+		publishedFiles[post.Slug+".md"] = true
+		if err := e.writePost(post); err != nil {
+			return err
+		}
+	}
+	if err := e.removeStalePosts(publishedFiles); err != nil {
+		return err
+	}
+	return nil
+}
+
+func (e *Exporter) removeStalePosts(publishedFiles map[string]bool) error {
+	entries, err := os.ReadDir(e.postsDir)
+	if err != nil {
+		return fmt.Errorf("read posts dir: %w", err)
+	}
+
+	for _, entry := range entries {
+		if entry.IsDir() || !strings.HasSuffix(entry.Name(), ".md") {
+			continue
+		}
+		if publishedFiles[entry.Name()] {
+			continue
+		}
+		if err := os.Remove(filepath.Join(e.postsDir, entry.Name())); err != nil {
+			return fmt.Errorf("remove stale post %s: %w", entry.Name(), err)
+		}
+	}
+	return nil
+}
+
+func (e *Exporter) writePost(post Post) error {
+	path := filepath.Join(e.postsDir, post.Slug+".md")
+	frontmatter := postFrontmatter{
+		ID:          post.ID,
+		Slug:        post.Slug,
+		Title:       post.Title,
+		Summary:     post.Summary,
+		Status:      string(post.Status),
+		Tags:        post.Tags,
+		Cover:       post.Cover,
+		Version:     post.Version,
+		SlugSource:  post.SlugSource,
+		SlugLocked:  post.SlugLocked,
+		PublishedAt: formatFrontmatterTime(post.PublishedAt),
+		CreatedAt:   formatFrontmatterTime(&post.CreatedAt),
+		UpdatedAt:   formatFrontmatterTime(&post.UpdatedAt),
+	}
+
+	var meta bytes.Buffer
+	encoder := yaml.NewEncoder(&meta)
+	encoder.SetIndent(2)
+	if err := encoder.Encode(frontmatter); err != nil {
+		return fmt.Errorf("encode frontmatter: %w", err)
+	}
+	if err := encoder.Close(); err != nil {
+		return fmt.Errorf("close frontmatter encoder: %w", err)
+	}
+
+	var output bytes.Buffer
+	output.WriteString("---\n")
+	output.Write(meta.Bytes())
+	output.WriteString("---\n\n")
+	output.WriteString(strings.TrimLeft(post.BodyMarkdown, "\n"))
+	if !strings.HasSuffix(output.String(), "\n") {
+		output.WriteByte('\n')
+	}
+
+	tmp := path + ".tmp"
+	if err := os.WriteFile(tmp, output.Bytes(), 0o644); err != nil {
+		return fmt.Errorf("write post %s: %w", post.Slug, err)
+	}
+	if err := os.Rename(tmp, path); err != nil {
+		return fmt.Errorf("replace post %s: %w", post.Slug, err)
+	}
+	return nil
+}
+
+func formatFrontmatterTime(value *time.Time) string {
+	if value == nil {
+		return ""
+	}
+	return value.Format(time.RFC3339)
+}